1. Field of the Invention
The present invention relates to compositions and methods of-producing color, information, and images by creating a material having microstructures which reflect a particular central color and bandwidth of light in accordance with their physical characteristics. More particularly, the invention relates to a material comprising periodic microstructures which reflect a particular central color and bandwidth of light in accordance with the periodicity of the microstructures, methods for fabricating the same, and applications of these periodic microstructures.
2. Background of the Art
Stepped microstructures for the reflection of specific bandwidths of visible light are known in nature, in particular in the tropical butterfly Morphos. The wing scales of these butterflies carry minute ridges that have corrugated surfaces consisting of a number of evenly spaced parallel plates separated by uniformly thick air gaps. The ridges consist of the complex carbohydrate chitin, which has a refractive index substantially greater than air. The parallel plates of chitin act like the high refractive index layers of a dielectric interference filter, while the air spaces between them act like the low index layers of a dielectric filter. The color and bandwidth of light reflected by these natural structures depends on a wide range of variables, including the physical dimensions and spacings of the chitin plates, the number of plates, the overall form of the microstructure, the presence or absence of pigment particles in the chitin, the refractive index of the chitin, and the orientation of the microstructures to both the light source and the viewer. The present invention differs from the color structures of the Morphos butterfly in a number of respects: the Morphos structure incorporates undercuts which prevent it from being an embossable shape and the Morphos color structure dimensions are limited to the reflection of blue-green light, while the structures of the subject invention can be designed to reflect light of any chosen color and bandwidth.
U.S. Pat. No. 5,407,738 to Tabata et al. discloses a microstructure which is identical in form and function to that of the Morphos butterfly. The microstructure is extrusion formed at a large scale in a polymer material and a second polymer layer is extruded around it, encasing it. The resulting thick fiber is heated and stretched to reduce its diameter, thereby reducing the size of the encased microstructure. Once the diameter of the microstructure has been reduced to the dimensions needed to provide optical function, the outer encasing layer is removed using a solvent, leaving the microstructured fiber exposed. This method enables the manufacture of an optically nonfunctional large structure and its subsequent reduction in scale to create a smaller optically functional structure. The methods of Tabata et al. are limited to the formation of a large structure and its subsequent reduction of its physical cross-sectional dimensions, and do not enable the formation of a small structure and subsequent expansion of the out-of-plane dimension of that structure to attain a different desired step spacing. Furthermore, the microstructure disclosed by Tabata et al. contains deep undercuts and it cannot be reproduced as an embossed surface. In addition, the microstructure of Tabata et al. requires air spaces between the plates, while the subject invention does not.
Periodic structures of other types have been also been used for the recording and reconstruction of color and color images. In 1895 Lippmann used incoherent interference effects to generate standing wave patterns within a silver-halide gelatin emulsion to create the first photographic recordings of color images. The Lippmann method involved the creation of a standing wave pattern within a silver emulsion by placing a reflective surface, typically mercury, in direct contact with an emulsion coated onto a glass plate. Ambient light from the scene was focused onto the emulsion through the glass plate and standing waves were generated by the interaction of the incoming and the reflected waves. The period of these standing wave patterns depends on the wavelength of the focused light. Monochromatic light produces a standing wave pattern with strong, distinct fringes, while a broader bandwidth produces a pattern which shows sharp, distinct fringe separations close to the reflecting interface and smoother, less distinct fringes further from the reflector. The softening of the fringes is the result of the creation of a continuum of fringe patterns, each having slightly different fringe spacing, from the different wavelengths of illumination. All of the fringes have an antinode at the reflector surface, so small differences in fringe spacing are not evident in that zone. The differences in fringe spacing accumulate the further the waves move from the reflector, reducing the contrast of the fringes. Once developed, this diffusing fringe pattern will selectively reflect the same bandwidth of illumination as was used to form it. Lippmann's standing wave fringe patterns were not rendered as a surface relief structure, in contrast with one of the primary objectives of the subject invention.
Bjelkhagen (Opt. Eng. 38(1) 55-61 (January 1999) New optical security device based on one-hundred-year-old photographic technique) discloses an application of the Lippmann process using a panchromatic photopolymer recording medium. In other respects Bjelkhagen's method is essentially the same as Lippmann's. Neither Lippmann nor Bjelkhagen disclosed any methods for altering the central color or the bandwidth reflected from their optical structures, nor any method for altering the spacing between their reflecting layers. The optical structures of Lippmann and Bjelkhagen are internal to the imaging medium; they do not exist as a surface relief structure and neither Lippmann nor Bjelkhagen disclosed any method for creating surface relief replicas from these internal optical structures.
It is known in the art to form stepped structures on the surface of a photosensitive material for the representation of holographic images, wherein the step heights of the structures are one-half the wavelength of the light, as measured within the photosensitive material, used to create the structures. U.S. Pat. No. 4,888,260 to Cowan teaches forming a volume phase reflection (VPR) hologram in a photosensitive material which is comprised of a phase relief stepped or terraced structure formed within another periodic structure. The distance between each step equals half the wavelength, in the photosensitive material, of the light from the beams which entered the medium from opposite directions to form the volume phase reflection hologram. The resulting volume phase reflection hologram is metallized and then overcoated with a high index material and the holographic image is reconstructed by illuminating the terraced phase hologram structure with a beam of light. The holographic image results from the constructive interference of light which is coherently back-scattered from the terraced structure at a wavelength, in air, equal to twice the step height multiplied by the index of refraction of the overcoating layer. The reconstructed back -scattered light has the same color as the recording beam if the overcoating layer has the same index of refraction as that of the original recording medium.
Cowan does not disclose or suggest producing a representation of a two-dimensional color image using stepped structures. Cowan is directed to converting a volume hologram into a surface relief hologram so that the hologram may be replicated by embossing. Moreover, Cowan does not disclose or suggest any method to adjust the dimensions of the stepped structure to accommodate the refractive index of an overcoating layer. Cowan teaches that a VPR hologram advantageously allows viewing at full parallax and in a single color. Col. 2, lines 55-59. Further, Cowan teaches that a full range of colors may be obtained with Cowan's structure by overcoating his structure with a layer of highly reflecting metal and then by overcoating the metal with appropriate clear dielectric layers. Col. 5, lines 28-32.
Neither Cowan nor Bjelkhagen teach or suggest the alteration of the step height to match the index and desired observation wavelength. Tabata et al. refers to multilayer structures that incorporate materials having different indices, not to stepped structures. Tabata discloses one method for reducing the layer spacing of multilayered color selective structures, but does not disclose any method for expanding the layer spacing. Neither of the methods of Tabata nor of Bjelkhagen is compatible with embossing processes. Neither the Tabata nor the Bjelkhagen structures can be formed as surface relief embossments.
Therefore, a need exists in the art for methods for: recording two-dimensional color image information as a microstructure in a master tool and generating embossments from the master which contain the two-dimensional color image information and which display the information in the visible light spectrum; producing pigment-like particles which are either single colored, plurality colored, or optically variably colored particles for inks, paints, polymers, papers, fabrics, and other coatings which are optically and chemically fade-resistant and which create the desired color effect by the interaction of light with a periodic microstructure; ‘printing’ of additive full-color images by providing a substrate which contains groups of color microstructures that produce a uniform color, or white, when viewed without artificial magnification, and which substrate can be acted on by optical, mechanical, thermal, or chemical means to modify, eliminate, or obscure the light intensity reflected from selected periodic microstructures so as to produce an image, pattern, or information representation from either the modified stepped microstructures or the unmodified stepped microstructures, or both; producing taggant devices based on periodic microstructure particles which selectively reflect chosen wavelengths of light. There also exists a need for light control materials including such microstructures.